a) Field of the Invention
The present invention relates to a method and apparatus for transmission of digital data, and more particularly to a digital image data transmitting method and apparatus suitable for transmission of the digital image data over a low-quality transmission line such as wireless communications network or the like by compressing the digital image data for transmission and expanding them after the transmission. More specifically, the present invention relates to a method and apparatus for transmission of digital image data, in which if an error has taken place or any of the data has been lost in the transmission of digital image data, the error or lost data can be interpolated with a substitution data.
b) Prior Art Statement
A typical conventional image data transmission system will be described with reference to FIGS. 24 through 28. FIG. 24 shows the configuration of the conventional image data transmission system, and FIGS. 25 and 26 show the flow charts, respectively, of the image data transmission.
As shown in FIG. 24, the image data transmission system includes a transmitter section composed of a source image encoder 2 for receiving and encoding an A-D converted input source image data 1, a forward error correction encoder 3 for forward error correction encoding of the coded data to minimize the influence of any transmission loss or error having taken place in the data during transmission over a transmission line 5, and a modulator 4 used to modulate the coded image data to a signal suitable for transmission over the transmission line 5. The source image encoding is effected independently of the error correction encoding, so that any part of the source image data of which any picture element block that could not be coded can be coded for error correction.
The image data transmission system also includes a receiver section composed of a demodulator 6 which demodulates a data received over the transmission line 5 from the transmitter section to provide the digital signal, an error correction decoder 7 for detection of any error in the demodulated signal, and a source image decoder 8 which receives from the decoder 7 the signal having been subjected to the error detection and decodes it to deliver an image data 14 which will be A-D converted to reconstruct the image data.
As shown in the flow chart of the image data transmission in FIG. 25, two kinds of data encoding, namely, the source image encoding and error correction encoding, are done independently of each other and judged serially to have been completed.
Also as shown in the flow chart of the image data reception in FIG. 26, the received data is decoded by reversely following the sequence of the image data transmission.
For the source image encoding, a highly efficient digital encoding is done for the band reduction. For this purpose, the standard ITU-TS T. 81 prescribed by JPEG (Joint Photographic Experts Group) of ITU or the transmission standard in ISO-IS010918, for example, is applied to partition a source image into blocks (MCU=Minimum code unit) each of 8.times.8 or 16.times.16 picture elements, have each MCU subjected to an adaptive discrete cosine transform (ADCT), quantize the transformed MCU through division by a quantizing constant (spectral quantizing) and to make a hybrid encoding (ADCT and Huffman encoding in combination) of the quantized MCU, thereby providing a data compressed in bits. In this case, attribute data including an image size, compression rate, compression method adopted, etc. are transmitted as added to the compressed coded data so that the coded data can be correctly decoded in the receiver section.
The data error correction will be briefly described herebelow. Image data is transmitted over any one of various transmission lines, wireless or wire. Such image data transmission has a possibility that a short break of the transmission line, noise, distortion or the like may have caused an error in the image data when received by the receiver section. Different from analog audio data and the like, digital image data includes little redundancy. For the transmission of digital image data, therefore, any such data error has to be detected and corrected to assure a constant quality of an image data reconstructed from such data, that is, such an extent of quality that the image data reconstructed after transmission can be read (legible).
One of the conventional error correction methods is such that a part of an image data in which an error has been detected is discarded and taken as lost data. The data is transmitted, received and reconstructed in disregard of the data error or with the data error removed from the entire data. Otherwise, the data is transmitted, received and reconstructed as it is while the data error is having an influence on a next data. In any case, an image data containing an error or short of a requisite part will be transmitted, received and reconstructed while having a kinds of adverse affect on a next data.
In case a low-quality wireless communications network is used as a transmission line in the above-mentioned conventional image data transmission system, however, since the source image encoding and forward error correction encoding are done independently of each other, a complex error correction encoding/request repeat system is required, which makes it difficult to accommodate a higher complexity and speed of the transmission apparatus and an increase of transmission time due to an increased amount of data to be transmitted. Moreover, there arises a problem that though errors can be decreased by the error correction encoding, a residual error, if any, will make it impossible to reconstruct the transmitted image.
In case it is requested to repeat the data transmission because an error has taken place in the preceding data transmission, the request for the repetition of data transmission will add to the transmitting procedure or the amount of data actually transmitted will be considerably larger than that normally transmitted, which will cause the transmission time to vary and an extra time for the re-transmission.
FIGS. 27(A) and 28(B) explain together the configuration of an image transmitted in the conventional image data transmission system; FIG. 27(A) shows an example of original image at the transmitter section and FIG. 27(B) shows an example of the image received by the receiver section. In the conventional source image encoding, a data is compressed in bits. If any error or lack exists in a data part a of MCU as shown in FIG. 27(B), it is impossible to correctly decode and reconstruct the image data even if the subsequent image data can be correctly received. Namely, the error will propagate to the subsequent image data in many cases.
FIGS. 28(A) and 28(B) explain an example of image data reconstruction by concrete possible images; FIG. 28(A) shows an original image and FIG. 28(B) shows a reconstructed image data. Especially if an error occurs in the attribute of an image data, the image cannot exactly be decoded and expanded as in FIG. 28(B). The incorrect reconstruction in FIG. 28(B) is resulted because the image size has not correctly been received due to a transmission error. In such case, all the data are to be transmitted again. However, the transmission time increases considerably, which will cause the data compression to be meaningless.
Moreover, in the conventional data error correction system, the picture element value of a lost area or an area containing an error is very different from that of an original image and such an area has an influence on and propagates to, subsequent data to be transmitted, so that the original image cannot be reconstructed with a high precision.